Polarization mode dispersion (PMD) is a polarization-related physical phenomenon that often limits the bandwidth-distance product of a fiber-optic-based telecom transmission link. In other words, PMD may be the primary impairment limiting the reach (i.e. maximum propagation distance) of high bit-/symbol-rate signals, and PMD may limit the bandwidth that may be carried in a single optical channel along a “long-haul” (i.e. long-distance) optical link.
Thus, it is desirable to be able to characterize one or more PMD-related parameters of an optical link, or of lightpaths thereof or of which they form a part.
Embodiments of aspects of the present invention enable characterization of different PMD-related parameters, and are particularly well suited to particular monitoring or measurement needs associated with optical networks. To facilitate detailed discussion and provide context for these embodiments, exemplary applications now will be presented.
1) Manifestation of PMD as a Function of Wavelength: DGD and “Partial DGD”
New optical networks will increasingly be based on a “mesh” topology or variants thereof, comprising multiple nodes connected by a corresponding multiplicity of fiber links. Such mesh networks may also employ Reconfigurable Optical Add-Drop Multiplexers (ROADMs) at the nodes to individually route any particular signal wavelength along a lightpath made up of different combinations of fiber links than the paths traversed by some of the other signal wavelengths. Signals at different wavelengths may carry payload at different bandwidths (e.g. 40 Gb/s instead of 10 Gb/s). The particular choice of route in the mesh may be determined by high-level control plane software, based on criteria such as:                (i) avoiding blocking or interference from other lightpaths having the same wavelength along certain node-to-node links in the network;        (ii) restoring a network after a cable cut along one or more such links; and        (iii) capacity of a particular lightpath to propagate the particular signal bandwidth without introducing an unacceptable level of impairments, e.g. arising from PMD.        
To this end, it may be advantageous to verify, using light from a test source, the differential group delay (DGD) at the wavelength of the particular lightpath shortly beforehand in order to ensure that the lightpath is suitable, i.e. there would not be excessive PMD-related impairment of the re-routed high-bandwidth data-carrying signal. Furthermore, the test source itself should not unduly perturb the network, e.g. affect the automatic gain control of the optical amplifiers, etc., or otherwise affect network operations.
The extent to which the PMD phenomenon induces temporal spreading of the symbols constituting a particular data-carrying signal (i.e. digital time slots, or “unit intervals”) at a given time is termed “partial DGD” (DGDP). DGDP is dependent upon both the wavelength and the SOP of the signal being launched into the optical fiber, in contrast to DGD, whose value is independent of the launched SOP. For a given optical frequency, the “worst-case” partial DGD corresponds to the DGD at that same optical frequency, which obtains when the magnitudes of the projection of the SOP of the signal onto each of the two (orthogonal) principal states of polarization (PSPs) at the input to the fiber are equal. In general, however, the signal SOP at the fiber input is not well controlled (and often may change unpredictably over time), and consequently the level of signal DGDP at a particular wavelength is less than the corresponding DGD at that same wavelength.
Measurement of partial DGD may prove advantageous in optical network troubleshooting, where an operator may need to determine whether sudden observed “bursts” in bit error rate (BER) may be PMD-related, rather than having been caused by other phenomena such as optical amplifier instabilities, self-phase modulation, intermittent connections, etc. As described by Boroditsky et at (US patent publication number 2005/0232640 A1), a correlation of the observed BER bursts with a sudden increase in DGDP, as monitored in real- or near-real time would indicate that PMD is indeed the likely cause of the problem. Furthermore, by carrying out such a DGDP measurement at points along the signal lightpath through the network (e.g. not necessarily limited to a particular optical link between two nodes of a mesh network), one may be able to approximately isolate the section primarily responsible for the PMD impairment.
2) PMD Determination of an in-Service Optical Link
If a telecom operator wishes to upgrade one or more channels of an existing in-service DWDM link by increasing the signal bandwidth (i.e. bitrate, including possibly a concomitant increase of the symbol rate), he may need to first verify whether the PMD of the existing optical link is sufficiently low to support transmission at this higher bandwidth. If measurements of the link PMD had been taken at some earlier date (e.g. during installation of the fiber plant), the documentation of these measurements may have been lost or misplaced, for instance following acquisition of existing fiber plant purchased from a third party. Furthermore, PMD values taken several years earlier may no longer be indicative of current PMD behavior, due to fiber ageing, etc.
Measurement methods and apparatus suitable for field characterization of fiber plant during initial installation, such as widely-used fixed-analyzer (or “wavelength scanning”) [1] and interferometric [2,3] methods, generally require a polarized broadband light source to launch test light, encompassing the spectral region of interest, into one end of the FUT and suitable receiver instrumentation at the opposite end. Obviously, the launch of a continuous spectrum of broadband light from a test source would likely disrupt network operations and, hence, would be incompatible with the concurrent transmission of active data-carrying signals in the same FUT. It would thus be desirable to be able to characterize PMD of an in-service optical link without disrupting the data-carrying signals.
In-service PMD may be determined by two approaches known in the art:                (i) DGD measurements in “dark channels” using test source(s): Polarized light from a test sources (e.g. broadband light, light from a tunable laser source) is launched into one end of the FUT, preferably at a multiplicity of wavelengths corresponding to respective “dark-channel” lightpaths, the wavelengths being distributed over a wide spectral region. At each wavelength, the test light is preferably launched with a multiplicity of substantially different SOPs. The resulting light is detected at the opposite end of the FUT—hence, this approach is classified as being “two-ended”. In this way, the DGD(υ) may be determined for each lightpath wavelength. As will be described in detail hereinbelow, this approach advantageously offers good accuracy;        (ii) DGDP measurements using “live” signal(s): In place of “test sources”, at least one, and preferably a multiplicity of widely-spaced data-carrying signals of the in-service network are used, the launched SOPs of which generally do not vary substantially with time. In this way, the DGDP(υ) or DGDP(υ,t) may be measured for each lightpath wavelength. As will be discussed hereinbelow, advantages are that test equipment need be placed only at one location (i.e. no test source is required) and that the measurement is completely “non-intrusive”, since light may be detected via existing tap couplers along the link. (Although a dedicated test source is not required in this approach, since the live data-carrying signals serve as “test sources”, it is convenient also to classify this approach as being “two-ended”, in opposition to “single-ended” OTDR-based to be described hereinbelow.)3) Single-Ended Measurement of Overall PMD of an Inactive Optical Link        
Prior art approaches for measurement of overall (i.e. “end-to-end”) PMD in an (unlit) optical link usually involve launching polarized light from a test source into one end of the FUT, detecting and analyzing the light exiting the FUT, and deducing the PMD therefrom using suitable analysis. However, there are significant additional operational costs involved with placing a dedicated source at one end and the measurement equipment at the other, in addition to the difficulties often associated with providing regular communication between the equipment placed at the opposing ends.
There is therefore a need for a single-ended measurement approach necessitating minimal intervention at the opposite end of the FUT, and which furthermore would be capable of measurement over the often 80 km or longer spans between optical amplifiers or ROADMs, etc.
4) Measurement of Cumulative PMD Along an Inactive Optical Link
As explained in commonly-owned U.S. Pat. No. 6,724,469 (Leblanc), in optical communication systems, an unacceptable overall polarization mode dispersion (PMD) level for a particular long optical fiber may be caused by one or more short sections of the optical fiber link. Where, for example, a network service provider wishes to increase the bit rate carried by an installed optical fiber link, say up to 40 Gb/s, it is important to be able to obtain a distributed measurement of PMD, i.e., obtain the PMD information against distance along the fiber, and locate the singularly bad fiber section(s) so that it/they can be replaced—rather than replace the whole cable.
Accordingly, Leblanc discloses a method of measuring distributed PMD which uses a polarization OTDR, to identify high or low PMD fiber sections, but does not provide a truly quantitative PMD value for the FUT. Consequently, because of its inherently “qualitative” nature, Leblanc's technique is not entirely suitable for development as a commercial single-ended overall PMD testing instrument that may measure the total PMD value for the entire of fiber link.
It is known to employ a so-called polarization-sensitive optical time domain reflectometer (POTDR; also commonly referred to as a “Polarization optical time domain reflectometer”) to try to locate such “bad” sections. Basically, a POTDR is an OTDR that is sensitive to the state of polarization (SOP) of the backreflected light. Whereas conventional OTDRs measure only the power of backreflected light to determine variation of attenuation along the length of an optical path, e.g., an installed optical fiber, POTDRs utilize the fact that the backreflected light also exhibits polarization dependency in order to monitor polarization dependent characteristics of the transmission path. Thus, the simplest POTDR comprises an OTDR having a polarizer between its output and the fiber-under-test (FUT) and an analyzer in the return path, between its photodetector and the FUT. (It should be appreciated that, although a typical optical transmission path will comprise mostly optical fiber, there will often be other components, such as couplers, connectors, etc., in the path. For convenience of description, however, such other components will be ignored, it being understood, however, that the term “FUT” used herein will embrace both an optical fiber and the overall transmission path to be characterized, according to context.)
A detailed review of the relevant prior art is provided in United States patent publication number US2010/0073667 A1 supra.
In order that the cumulative PMD may be characterized along fiber lengths commonly used in installed systems, typically spanning the 60-80 km distances between optical amplifiers, ROADMs, etc. in optical networks, the POTDR should have a dynamic range sufficient to characterize at least half the end-to-end fiber length. The other half may then be characterized by repeating the measurement process from the other end of the link, and the data from each end may be “stitched” together to provide a full cumulative PMD profile along the link.
(In this specification, the term “cumulative PMD” is used to distinguish from the aforementioned “overall” PMD that is traditionally measured from end-to-end. Because PMD is not a localized quantity, PMD(z) is an integral from 0 to z, bearing resemblance to a cumulative probability rather than the probability distribution. When distance z is equal to the overall length of the FUT, of course, the cumulative PMD is equal to the overall PMD.)